Touch sensors are increasingly popular devices because they provide a natural interface between an electronic system and operator. Rather than using a keyboard to type in data, for example, touch sensors allow the user to transfer information to a computer by touching a displayed icon, or by writing or drawing on a screen. It is desirable in many applications for the touch screen to be transparent and positioned over a display.
Several types of transparent touch sensors use resistive or capacitive techniques to detect touch location. A resistive touch sensor includes two layers of transparent conductive material, such as a transparent conductive oxide, separated by a gap. When touched with sufficient force, one of the conductive layers flexes to make contact with the other conductive layer. The location of the contact point is detectable by controller circuitry that senses the change in resistance at the contact point.
Resistive touch sensors depend upon actual contact between the conductive layers, and this technique presents several problems. First, frequent contact causes the conductive layers to rub and may damage the conductive layers. Transparent conductive layers are brittle and flexing causes degradation in the conductive material due to microcracks at the flex point. With repeated flexing, an area of conductive material may flake off from one contact, thereby causing voids in the contact layers. Also, the conductive layer used in a resistive touch sensor may degrade due to interaction between the conductive layers and surrounding materials. Degradation of the conductive layers by the mechanisms discussed above may result in decreased optical transmission or loss of sensitivity, resolution, and accuracy in detecting the touch location.
A capacitive touch sensor typically includes a single conductive layer for touch detection. A finger touch to the sensor provides a capacitively coupled path from the conductive layer through the body to earth ground. The location of the contact point is detectable by controller circuitry that measures a change in a capacitively coupled electrical signal at the touch location.
In contrast to the resistive touch sensor, operation of the capacitive touch sensor described above does not require flexing a conductive layer to the point of contact. However, the capacitive sensor requires a more complex controller to compensate for low signal to noise ratio due to large external capacitance and the variability of the capacitance of the finger touch. A capacitive touch sensor requires that the touch produce a capacitively coupled circuit to ground and is only operable by a finger touch or a conductive stylus. Touches with a non-conductive stylus, gloved hand or fingernail are typically undetectable, or are undetectable under most circumstances. Further, operating a sensor by a finger touch may not provide the spatial resolution required for some applications, for example, drawing on a small hand held computer screen. Spatial resolution may be improved using a conductive stylus, however, some users find the requirement of a special stylus objectionable.